Most electronic systems and devices contain circuits, logic and storage elements, e.g., memory, which have indeterminate states when the primary power source for the system is first applied, or when the power source drops below some minimum operating level. The circuits, logic and storage elements, e.g., memory devices, are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory, among others. Memory devices are utilized for a wide range of electronic applications including personal computers, personal digital assistants (PDAs), digital cameras, cellular telephones, etc.
Incorrect and/or unreliable data can be read from the circuits, logic and storage elements, e.g., memory, during power up due to the fact that the supply voltage of the device is ramping from zero volts to a VCC level. An incorrect read operation can result in operational errors such as erroneous redundancy address selection or erroneous trimming operations, failure to boot, etc. Therefore, it is desirable and often necessary, to provide some means whereby the storage elements are set to a known state at initial power on or after a power drop. Such circuits are sometimes referred to as power-on reset (POR) circuits.
POR circuitry is often used in memory devices to insure proper functionality of the device when power is initially applied to the device, e.g., during power on of the device, and to insure proper functionality of the device if power to the device is temporarily lost. Power-on reset circuits can prevent various internal circuits of the memory device, e.g., logic circuits, processors, latches, charge pumps, and voltage regulators, among others, from functioning until after the POR circuit determines that the applied supply voltage, e.g., Vcc, is adequate to insure proper circuit function.
A wide variety of internal circuits are dependent on POR supervision of their functionality with respect to available voltage supply. The various circuits within a given electronic device or system can have differing acceptable voltage supply requirements. In previous approaches, either one voltage threshold was selected that satisfied the voltage supply requirements of all dependent circuits delaying power-up of some circuits with lower acceptable voltage thresholds, or multiple PORs were applied to supervise the multiple voltage supply thresholds, using more circuit real estate and increasing costs.
One difficulty in implementing POR circuits is that such circuits are often be powered by the same voltage source that is monitored by the circuit. This can present a challenge, particularly if the circuit is used to ensure that the system is in a proper initial state at relatively low supply voltages. Furthermore, POR circuits should operate reliably when the input supply voltage either has a very fast rise time or a slow rise time. Additionally, the electronic deices and systems of today operate in a wide range of temperature environments. As such, POR circuits should be able to function accurately in determining voltage supply suitability for the circuits they supervise over a range of temperature variations.